The present invention relates to chemical mechanical polishing of workpieces. In particular, the present invention relates to a workpiece handoff station for staging workpieces between processing stations, the handoff station including a workpiece processing surface.
Recent rapid progress in semiconductor device integration demands smaller and smaller wiring patterns or interconnections, and narrower spaces between interconnections which connect active areas. One of the processes available for forming such interconnections is photolithography. Though the photolithographic process can form interconnections that are at most 0.5 microns wide, it requires that surfaces on which pattern images are to be focused by a stepper be as flat as possible because the depth of focus of the optical system is relatively small.
It is therefore necessary to make the surfaces of semiconductor wafers flat for photolithography. One customary way of flattening the surfaces of semiconductor wafers is by Chemical Mechanical Planarization (CMP), which is a process whereby semiconductor wafers are polished with a polishing apparatus.
Conventionally, a CMP polishing apparatus has a turntable and a wafer carrier which rotate at respective individual speeds. A polishing pad is attached to the upper surface of the turntable. A semiconductor wafer seated in the carrier is lowered into engagement with the polishing pad, and clamped between the carrier and the turntable, typically through the exertion of downward force by the carrier. An abrasive grain containing liquid (known as slurry) is deposited onto the polishing pad and retained on the polishing pad. During operation, the carrier exerts a certain pressure on the turntable, and the surface of the semiconductor wafer held against the polishing pad is therefore polished by a combination of chemical polishing and mechanical polishing to a flat mirror finish while the carrier and the turntable are rotated.
The semiconductor wafer that has been polished carries abrasive liquid and ground-off particles attached thereto. Therefore, after polishing, the semiconductor wafer is cleaned and dried in one or more cycles and then housed in a clean storage cassette. If the wafer is not cleaned immediately, the slurry and foreign particles applied to the lower surface of the wafer tend to solidify, becoming very difficult to remove. Also, the known standard cleaning processes, employing, for example, roller brush box type cleaners, are largely ineffective at removing submicron scratches left on the wafer surface by the polishing process.
Thus, additional processing is typically done prior to the wafer cleaning step. For example, a second polish turntable with a second carrier may be employed, using a relatively soft buffing pad in combination with a cleaning chemical, or ultra pure water alone. The buffing process can be effective at removing the residual slurry and buffing out the surface scratches left from the polishing process before cleaning the wafer. However, the effectiveness of the buffing process is also affected by the length of time that slurry sits on the wafer between the polish and buffing process. Unfortunately, adding the buffing process necessitates additional wafer handling and transferring capability, increased tool foot print, and often reduced wafer throughput as a result.
Alternatively, the slurry and surface scratches may be removed through use of a Hydrofluoric (HF) acid etching process. In such a process, the wafer may be dipped in a bath of the HF acid solution and/or cleaned with an HF solution in a somewhat conventional brush box. However, HF acid poses serious health risks. Compliance with industry safety standards governing the use of HF acid adds substantially to the cost of the equipment and the facility which houses the equipment when employing these techniques.
It is an object of the present invention to provide a method and apparatus for cleaning post polish slurry residue from the surface of a wafer without allowing time for the residue to significantly solidify.
It is another object of the present invention to provide a method and apparatus for buffing a wafer to remove post polish defects that minimizes the time between polishing and buffing and does not increase tool footprint.
It is still another object of the present invention to provide an alternative solution to HF acid etch for pre-cleaning removal of wafer surface particles and defects without employing a conventional buffing table.
The present invention achieves these objects by providing a dual purpose workpiece handoff station for intermediately staging a semiconductor wafer (or other workpiece) being transferred between processing stations in a CMP machine. The handoff station includes a workpiece processing surface such as a polishing pad or buffing pad which includes a plurality of apertures for applying fluids to the surface of a workpiece. A fluid delivery system is provided for selectively delivering water, chemicals, or slurry, for cleaning and polishing. In addition, the delivery system may provide vacuum for holding a wafer, or nitrogen for wafer blowoff.
In operation, a workpiece carrier moves a polished workpiece from a primary polishing surface to the handoff station, and polishes, buffs, or cleans the workpiece in the handoff station by rotating the workpiece and oscillating the workpiece across the handoff station polishing surface while pressing the workpiece thereon. Cleaning or buffing chemicals may be simultaneously applied to the workpiece. A robot, preferably track mounted, retrieves the wafer from the handoff station and transfers it to a subsequent station, for example to a second primary polish station, or to a cleaning station.
These and other objects, features and advantages of the present invention are specifically set forth in, or will become apparent from, the following detailed description of a preferred embodiment of the invention when read in conjunction with the accompanying drawings.